OK, so let's say I want to make a Doctor Who game. I want the TARDIS to actually appear to be bigger on the inside, not just a lame teleport or an 8-bit picture. I want a box that is actually bigger on the inside. So, IS it possible? PS I don't know what to put so I put camera.
Yes, it is possible, but you won't be able to see both at the same time (From the outside looking in).
You could put parts into the CurrentCamera, having the inside being visible when inside a certain radius, and having the outside model visible when outside that radius. However, that would get really messy, and would be hard to implement properly.
In ROBLOX, your best bet would be a "lame teleport", unfortunately.